This invention relates to a method and system for amplifying two or more radio frequency signals having a specific temporal relationship in such a way that said relationship is not significantly affected. Although it is of more general applicability, a principal use is in a modification of the electronic counter measures (ECM)system known as xe2x80x9ccross-eyexe2x80x9d, and its application to such systems will be specifically described.
In the xe2x80x9ccross-eyexe2x80x9d system, radar signals are received by two antennas located some distance apart on the target. One of the received signals is shifted 180xc2x0 in phase, the two signals are amplified, and re-transmitted. Because the two transmitted signals are 180xc2x0 out of phase, when they are received by the hostile radar receiver they cancel each other out at the focal point, leaving a xe2x80x9cholexe2x80x9d where the point of maximum strength would ordinarily be perceived. In addition, the relative amplitude of the two re-transmitted signals may be varied for further deception.
To make xe2x80x9ccross-eyexe2x80x9d an effective technique, it is important, among other things, that the relationship in time between the signals received at the two target antennae be preserved precisely. If two amplifier systems are used, this is not possible, as a practical matter, because of inherent differences between two amplifier systems no matter how carefully they are constructed. An attempt has been made to circumvent this difference by time sharing the same amplifier. Two such systems are described in my copending application Ser. No. 05/024,933, filed Apr. 1, 1970. However, one of the systems there described is subject to isolation problems caused by reflections in the delay line. The other introduces a small time difference in the transmission of the two signals. While this is acceptable if one is dealing with radars having narrow processing bands its applicability is limited to such narrow band systems and it is ineffective with other systems.
According to the present invention, the limitations inherent in the previous time sharing systems referred to are overcome and the received signals may be amplified with adequate isolation and retransmitted (with a phase change for one signal, as desired) without significantly changing the temporal relationship between them. To accomplish these ends, the invention in its broadest aspect provides a system comprising a first delay means, means for delivering a first signal to said first delay means, a second delay means, means for delivering a second signal to said second delay means, means for delivering said first signal from said first delay means to said second delay means after said second signal has entered said second delay means, signal amplification means connected to said second delay means and switching means for alternately maintaining the output from said amplification means connected to said first delay means while said second signal is emerging from said amplification means, and for maintaining the output from said amplification means disconnected from said first delay means while said first signal is emerging from said amplification means, whereby the amplified second signal may be recovered from said first delay means and the amplified first signal may be recovered from said amplification means without affecting significantly their temporal relationship. In an-ECM system said signals may be taken from antenna means, and retransmitted through the antenna means, the first signal being transmitted through the antenna used to receive the second signal and vice versa. Phase reversal means for one signal may be provided.